Discussion on dispensing technology and technical requirements of SMT chip processing
The through-hole assembly (THT) and surface mount (SMT) of lead components are the most common assembly method in the current electronic product production. In the whole production process, one component of a printed circuit board (PCB) can only be welded by wave soldering from the beginning to the end after glue dispensing and solidification. The interval between them is long, and there are many other processes, so the solidification of components is particularly important, so it is of great significance for the research and analysis of the glue dispensing process.
1、 SMT chip processing glue and its technical requirements:
The glue used in SMT is mainly used in the wave soldering process of chip components, sot, SOIC and other surface mount devices. The purpose of fixing surface mounted components on PCB with glue is to avoid falling off or displacement of components under the impact of high-temperature wave crest. In general, epoxy resin heat curing glue is used in production instead of acrylic acid glue (UV curing is required).
2、 SMT work requirements for patch glue:
1. The glue should have good thixotropic property;
2. No wire drawing;
3. High wet strength;
4. No bubbles;
5. Low curing temperature and short curing time of glue;
6. It has enough curing strength;
7. Low moisture absorption;
8. It has good repair characteristics;
9. No toxicity;
10. The color is easy to identify, so as to check the quality of glue points;
11. Packaging. The packaging type shall be convenient for the use of the equipment.
3、 Process control plays an important role in the dispensing process.
The following process defects are easy to appear in production: unqualified glue point size, wire drawing, glue impregnated pad, poor curing strength, easy to drop pieces, etc. In order to solve these problems, we should study all kinds of technological parameters, so as to find the solution.
1. Size of dispensing amount
According to the work experience, the diameter of the glue point shall be half of the pad spacing, and the diameter of the glue point after mounting shall be 1.5 times the glue point diameter. This will ensure that there is enough glue to bond the components and avoid too much glue to dye the pad. The amount of dispensing depends on the rotation time of the screw pump. In practice, the rotation time of the pump should be selected according to the production conditions (room temperature, glue viscosity, etc.).
2. Dispensing pressure (back pressure)
At present, the glue dispenser adopts a screw pump to supply the glue dispensing needle with a pressure to ensure enough glue to supply the screw pump. If the backpressure is too large, it is easy to cause glue overflow and too much glue; if the pressure is too small, it will cause intermittent glue dispensing and leakage, thus causing defects. The pressure should be selected according to the glue of the same quality and the working environment temperature. If the ambient temperature is high, the viscosity of the glue will be reduced and the fluidity will be better. At this time, it is necessary to lower the back pressure to ensure the supply of glue, and vice versa.
3. Needle size
In practice, the inner diameter of the tip should be 1 / 2 of the diameter of the dispensing point. In the process of dispensing, the tip should be selected according to the size of the pad on the PCB: for example, the size of the pads of 0805 and 1206 is similar, and the same tip can be selected, but for the pads with a great difference, different tips should be selected, which can not only ensure the quality of the glue point but also improve the production efficiency.
4. Distance between needle and PCB
Different dispensing machines use different needles, some of which have a certain degree of stop (such as cam / a lot 5000). At the beginning of each work, the distance between the needle and PCB shall be calibrated, i.e. z-axis height calibration.
5. Glue temperature
The general epoxy resin glue shall be stored in the refrigerator of 0-50c, and shall be taken out 1 / 2 hour in advance when used, so as to make the glue fully conform to the working temperature. The use temperature of the glue should be 230c-250c; the ambient temperature has a great influence on the viscosity of the glue, and if the temperature is too low, the glue point will become smaller, resulting in wire drawing. The difference in ambient temperature is 50c, which will cause a change of 50% dispensing amount. Therefore, the ambient temperature should be controlled. At the same time, the temperature of the environment should also be guaranteed, the small humidity of the glue point is easy to dry, affecting the adhesion.
6. Viscosity of glue
The viscosity of glue directly affects the quality of dispensing. If the viscosity is large, the glue point will be smaller, or even drawn; if the viscosity is small, the glue point will be larger, and then the pad may be stained. In the process of dispensing, the proper back pressure and dispensing speed should be selected for different viscosities of glue.
7. Curing temperature curve
For the curing of glue, the general manufacturer has given the temperature curve. In practice, a higher temperature should be used as much as possible to make the glue have enough strength after curing.
8. bubbles
There must be no air bubbles in the glue. A small gas will cause many pads to have no glue; when changing the rubber tube in the middle of each time, the air at the joint shall be emptied to prevent airstrike.
For the adjustment of the above parameters, it should be done in the way of point and surface. The change of any parameter will affect other aspects. At the same time, the occurrence of defects may be caused by multiple aspects. The possible factors should be checked item by item and then eliminated. In a word, the parameters should be adjusted according to the actual situation in the production to ensure the production quality and improve production efficiency.
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